SDH and SONET are both examples of protocols suitable for use in sending data over synchronous transmission networks. Such networks comprise a plurality of network elements (also referred to as nodes) connected together by optical fibre or any other suitable type of connection. Each node comprises a synchronous transmission apparatus, or network equipment, in the form of, for example, a multiplexer, a cross-connect, a regenerator, or other signal processing or switching apparatus. The synchronous transmission apparatus, or network elements, incorporated in the network are typically synchronised (locked in frequency but not in phase) to an overall network clock.
The SONET (Synchronous Optical Network) Standard defined by ANSI (American National Standards Institute) is similar in many respects to the SDH (Synchronous Digital Hierarchy) standard defined by the ITU-T (International Telecommunications Union Telecommunications Standardization Sector). One key difference, however, is the respective basic unit of payload. In SONET, the basic unit is an STS-1, which offers a bandwidth of approximately 50 Mbit/s. With SDH, the basic unit is an AU-4, which offers a bandwidth of approximately 150 Mbit/s. SDH channels with smaller bandwidths are available and are provided by, for example, basic units known as TU-12 (approximately 2 Mbit/s) and TU-2 (approximately 6 Mbit/s). However, TU-12s and TU2s are carried within VC-4s which, in turn, are carried within AU-4s. The TU-12s and TU-2s are therefore referred to as low order data structures, the VC-4 being a high order data structure.
In SONET an STS-1 can be used to carry equivalent low order structures known as Virtual Tributaries (VTs) where a VT2 is equivalent to a TU-12 and a VT6 is equivalent to a TU-2.
The temporary conversion of payloads from SDH to SONET and back to SDH within a node may also be regarded as a conversion from the de-facto preferred SDH multiplexing structure, based on the AU-4, to the non-preferred structure, based on the AU-3, and back again. The SDH AU-3 is structurally identical to a SONET STS-1, but the meanings associated with some bits and bytes in the two payload types are different. For clarity, this description will always refer to the converted data as SONET.
It is advantageous to provide a synchronous transmission network which is operable for both SDH and SONET in order to provide greater flexibility. It is also desired to provide a network node which can be used in either a SONET only network or an SDH only network, but without increasing the complexity and cost of the network node.
However, it is also advantageous to use nodes in such a synchronous transmission network which have a single protocol switch core. This simplifies the switch cores and so keeps development and manufacture costs down. However, previously it has not been possible to provide such a node with a single-protocol switch core that can be used in either an SDH only or a SONET only network.
The invention seeks to provide an improved method and apparatus for converting between SONET and SDH which overcomes or at least mitigates one or more of the problems noted above.
The invention also seeks to provide a network node for a synchronous transmission network, whereby the network node is able to carry both SDH and SONET traffic whilst having a single protocol switching core.
An object of the invention is to provide a synchronous transmission network comprising a plurality of nodes each having a single protocol switch core and which overcomes or at least mitigates one or more of the problems mentioned above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.